Fuel cells convert the chemical energy in fuels such as alcohols and hydrocarbons into usable electricity at efficiencies higher than those obtained by conventional thermal combustion. Such conversion is accomplished without production of pollutants such as SOn, NOn, and carbon soot. Since fuel cells operate at optimal efficiencies when H2 is used as a fuel, there is currently a worldwide effort to develop and refine means of generating hydrogen from conventional fuels. These efforts include developing on-board automobile catalytic fuel processors that will generate hydrogen from gasoline or diesel fuel in a manner compatible with existing vehicle fuel distribution networks.
Conventionally, H2 is produced in the chemical industry (for the manufacture of NH3 or methanol, for example) from hydrocarbons by way of a four-stage process. First, hydrocarbon feedstock is hydrodesulfurized to less than 0.5 ppm of S using sulfided CoMo or NiMo catalysts and a ZnO. Second, the feedstock is subjected to steam reforming with excess steam at 800-1000° C. pursuant to the following (highly endothermic) reaction:CH4+H2O→CO+3H2 that uses Ni catalysts supported on alumina, magnesia, silica, or calcium aluminate. The H2 and CO product of this reaction is referred to as synthesis gas. Next, the CO generated in steam reformation is subjected to a two-step, exothermic water gas shift reaction at 200-450° C. pursuant to the following reaction:CO+H2O→CO2+H2 that utilizes a high temperature shift reaction employing Fe-based catalysts and a low temperature shift reaction on Cu—ZnO catalysts. Finally, the CO content of the steam reforming reaction effluent is reduced to about 1 ppm by methanation according to the following reaction:CO+3H2→CH4+H2Ousing nickel-based catalysts or by partial oxidation (CO+0.5O2→CO2) using Pt-based catalysts such as Pt—CeO2.
Alternatively, exothermic partial oxidation of alkanes or other hydrocarbon-containing feedstocks can be used to generate H2 in a much lower ratio of H2 to CO. Partial oxidation occurs pursuant to the following reaction, using methane as a feedstock example:CH4+0.5O2→2H2+COCO+0.5O2→CO2 The most active catalysts for steam reforming or partial oxidation of hydrocarbons usually contain nickel. However, conventional steam reforming or partial oxidation catalysts based on Ni (such as Ni—Al2O3, Ni—MgO, Ni—Ca—Al2O4, Ni—SiO2 etc) lack sufficient activity for conversion of CO to CO2. Although nickel on alumina catalysts are effective for the conversion of methane to synthesis gas using molecular oxygen, such a catalyst, as well as commercial nickel-containing steam reforming, steam cracking, and partial oxidation catalysts, form coke and deactivate relatively rapidly. While transition metal catalysts, such as ruthenium on alumina, can be used to reform a hydrocarbyl compound in the presence of molecular oxygen, such transition metals are expensive.
One disadvantage of known catalysts and processes for the generation of hydrogen is that the hydrocarbon or alcohol feedstock that is used must be desulfurized to a level of less than 0.5 ppm of sulfur-containing compounds. This is because in the presence of such compounds, prior art catalysts undergo severe deactivation leading to drastic reduction in their productivity, selectivity and durability. The desulfurization of hydrocarbon feedstocks to a level of below 0.5 ppm of sulfur compounds prior to their use in hydrogen production processes such as steam reforming, autothermal reforming, water gas shift reaction and partial oxidation is expensive and increases the cost of the hydrogen generated by such processes.
Thus, there is a significant interest in improving the efficiencies and yields of processes that generate hydrogen, for example by reforming hydrocarbon feedstocks such as gasoline, diesel fuel, natural gas, or other fuel sources such as alcohol. Economically improving such efficiencies and yields calls for an affordable, durable, sulfur-tolerant, coke-resistant, highly active, and selective hydrogen generation catalyst. In particular, fuel cells require active, multi-functional catalysts that (1) can operate at lower temperatures; (2) facilitate the aforementioned hydrogen generation reactions; and (3) enable a more compact fuel processor design.
To meet the power needs of hydrogen-oxygen fuel cells, a hydrogen generation catalyst employed in a fuel processor must be able to generate H2 from a hydrocarbon fuel containing typical quantities of sulfur compounds at acceptable rates and operating temperatures. Ideally, the catalyst must perform over extended periods of time and in a relatively short start-up time. The activity of the preferred catalyst must be such that it generates a gas sufficiently rich in hydrogen in a relatively small fuel processor. Among numerous potential applications of this catalyst, a current automotive design objective is the production, within 30 seconds of start-up, of 50 kW of fuel cell power derived from a 7 liter fuel processor. The fuel processor could be an integral packed bed catalytic reactor capable of generating, within the aforementioned start-up times, H2 in sufficient yields, and at acceptable temperatures, to meet vehicular power requirements.
In designing a fuel processor for generation of hydrogen, the effect of catalyst type and configuration on steam reforming or partial oxidation reactor design and performance must be considered. Variation in catalyst type, volumetric density, and dispersion within a reactor bed can lead to increased pressure drop.
Ashcroft et al., Nature, Volume 352, page 225, (1991), describes the reforming of methane with carbon dioxide to form synthesis gas, a mixture of CO and hydrogen, using catalysts such as palladium, ruthenium and iridium on alumina, as well as nickel on alumina.
In U.S. Pat. No. 3,791,993, catalysts containing nickel for reforming gaseous or vaporizable liquid hydrocarbons using steam, carbon oxide, oxygen and/or air were prepared by coprecipitating a nickel salt, a magnesium salt and an aluminate to form a sludge. The sludge was then washed until substantially free of sodium and potassium, dried, and dehydrated at 300° C. to 750° C. The ultimate catalyst was formed after calcination at 850° C. to 1100° C. Examples show that compositions having a 1:1:2 or a 2:7:1 mole ratio of nickel, magnesium and aluminum, respectively, are suitable for converting naphtha to hydrogen-rich gaseous products using steam reforming.
U.S. Pat. No. 6,162,267 discloses steam reforming catalysts that include nickel with amounts of noble metal, such as cobalt, platinum, palladium, rhodium, ruthenium, iridium, and a support such as magnesia, magnesium aluminate, alumina, silica, zirconia, singly or in combination. These catalysts can be a single metal such as nickel or a noble metal supported on a refractory carrier such as magnesia, magnesium aluminate, alumina, silica, or zirconia, singly or in combination, promoted by an alkali metal such as potassium. Nickel supported on alumina and promoted by an alkali metal such as potassium is preferred.
Redox active transition metal oxides are well known as components of commercial catalysts. Such oxides are typically incorporated by impregnation on a support or co-precipitation to form a bulk catalyst. Examples are found in Catalytic Air Pollution Control, Commercial Technology, 2nd Ed. 2002, R. M. Heck and R. J. Farrauto, John Wiley and Roh et al., Cat.Lett., Vol. 74, p. 31, 2001.
An example of a catalyst with a single component “two-dimensional” redox active metal oxide monolayer has been reported by Putna et al., Cat. Today, Vol. 50, p. 343, 1999. An example of a two-component, monolayer of metal oxide has been reported by Gampine et al., J Cat., Vol. 179, p. 315, 1998 (“Gampine”). Gampine does not disclose catalysts comprising both a redox inactive and a redox active component within a monolayer of metal oxide and does not disclose a monolayer comprised of a redox inactive and a redox active metal oxide as a component of an active catalyst phase. The TiO2 and ZrO2 used in the monolayer employed in Gampine's catalysts are both redox inactive metal oxides.